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Fair Value Measurement (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Fair Value Measurement [Line Items] |  |
Assets | $ 3,403,822 mcy_FairValueAssetsMeasuredOnRecurringBasis | $ 3,159,962 mcy_FairValueAssetsMeasuredOnRecurringBasis
Liabilities | 4,219 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis | 140 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 742,886 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 558,750 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Liabilities | 194 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 140 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,649,217 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 2,584,362 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Liabilities | 4,025 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 11,719 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 16,850 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Liabilities | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Short-term bonds [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 88,361 mcy_FairValueAssetsMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember | 52,888 mcy_FairValueAssetsMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember
Short-term bonds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 69,999 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember | 39,998 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember
Short-term bonds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 18,362 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember | 12,890 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember
Short-term bonds [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_ShortTermInvestmentsMember
Money market instruments [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 284,181 mcy_FairValueAssetsMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember | 1,650 mcy_FairValueAssetsMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember
Money market instruments [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 284,181 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember
Money market instruments [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember | 1,650 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember
Money market instruments [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MoneyMarketFundsMember
Total Return Swap [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 4,025 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_TotalReturnSwapMember |
Total Return Swap [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_TotalReturnSwapMember |
Total Return Swap [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 4,025 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_TotalReturnSwapMember |
Total Return Swap [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_TotalReturnSwapMember |
Equity contracts [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 194 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember | 140 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember
Equity contracts [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 194 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember | 140 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember
Equity contracts [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember
Equity contracts [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember | 0 mcy_FairValueLiabilitiesMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_EquityContractMember
Interest rate swap agreements [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets |  | 262,888 mcy_FairValueAssetsMeasuredOnRecurringBasis / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_InterestRateSwapMember
Interest rate swap agreements [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets |  | 262,888 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_InterestRateSwapMember
Interest rate swap agreements [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets |  | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_InterestRateSwapMember
Interest rate swap agreements [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets |  | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_InterestRateSwapMember
Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,108 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 16,096 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,108 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 16,096 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fixed Maturity Securities [Member] | Municipal securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,275,455 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember | 2,235,323 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember
Fixed Maturity Securities [Member] | Municipal securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember
Fixed Maturity Securities [Member] | Municipal securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,275,455 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember | 2,235,323 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember
Fixed Maturity Securities [Member] | Municipal securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MunicipalBondsMember
Fixed Maturity Securities [Member] | Mortgage-backed securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 47,691 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 40,247 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fixed Maturity Securities [Member] | Mortgage-backed securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fixed Maturity Securities [Member] | Mortgage-backed securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 47,691 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 40,247 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fixed Maturity Securities [Member] | Mortgage-backed securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fixed Maturity Securities [Member] | Corporate Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 256,930 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember | 264,685 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember
Fixed Maturity Securities [Member] | Corporate Securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember
Fixed Maturity Securities [Member] | Corporate Securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 256,930 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember | 264,685 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember
Fixed Maturity Securities [Member] | Corporate Securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_CorporateDebtSecuritiesMember
Fixed Maturity Securities [Member] | Collateralized debt obligations [Member] |  |
Fair Value Measurement [Line Items] |  |
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Fixed Maturity Securities [Member] | Collateralized debt obligations [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Fixed Maturity Securities [Member] | Collateralized debt obligations [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Fixed Maturity Securities [Member] | Collateralized debt obligations [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Public utilities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 105,485 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_PublicUtilityEquitiesMember | 85,287 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_PublicUtilityEquitiesMember
Equity Securities [Member] | Public utilities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 105,485 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_PublicUtilityEquitiesMember | 85,287 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_PublicUtilityEquitiesMember
Equity Securities [Member] | Public utilities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Public utilities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Banks, trusts and insurance companies [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 9,757 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember | 2,927 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember
Equity Securities [Member] | Banks, trusts and insurance companies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 9,757 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember | 2,927 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember
Equity Securities [Member] | Banks, trusts and insurance companies [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Banks, trusts and insurance companies [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember | 0 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember
Equity Securities [Member] | Energy and other [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 257,356 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember | 151,554 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember
Equity Securities [Member] | Energy and other [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 257,356 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember | 151,554 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember
Equity Securities [Member] | Energy and other [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Energy and other [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Non-redeemable preferred stock [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 28,563 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_NonredeemablePreferredStockMember | 29,567 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_NonredeemablePreferredStockMember
Equity Securities [Member] | Non-redeemable preferred stock [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Non-redeemable preferred stock [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 28,563 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_NonredeemablePreferredStockMember | 29,567 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = us-gaap_NonredeemablePreferredStockMember
Equity Securities [Member] | Non-redeemable preferred stock [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Partnership interest in a private credit fund [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 11,719 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = mcy_PartnershipInterestInPrivateCreditFundMember | 12,548 mcy_FairValueAssetsMeasuredOnRecurringBasis / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_EquitySecuritiesMember / mcy_FairValueQualitativeDisclosuresRelatedToElectionByEligibleItemOrGroupAxis = mcy_PartnershipInterestInPrivateCreditFundMember
Equity Securities [Member] | Partnership interest in a private credit fund [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Partnership interest in a private credit fund [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
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Equity Securities [Member] | Partnership interest in a private credit fund [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
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